a childish action or prank; childish behaviour ...1827
from French s'enfantiller (childishness), from enfant (child)
FIRST DOCUMENTED USE
1827 - see EXAMPLE below
"...Soult and Oudinot, Duc de Reggio and Duc de Dalmatie, have sent back their invitations, and many both ultra and liberals, fifty peers and all that belong to the army, have agreed they will not go to the house.
I think it is an enfantillage, a bêtise, on the part of the Austrian Government, and most natural of the Marshals and their wives not to go to Appony's...."
From: Letters of Harriet, Countess Granville, 1810-1845
Edited by Her Son The Hon. F. Leveson Gower (1894)
To Lady Carlisle, Paris, 1827